Low-substituted hydroxypropyl cellulose insoluble in water but soluble in an aqueous alkali solution is produced in a method of partially dissolving the low-substituted hydroxypropyl cellulose, by partially neutralizing, in water, alkali hydroxide used as a reaction catalyst after completion of an etherification reaction (Japanese Patent Application Examined Publication No. 57-53100).
In a purification step for obtaining water-soluble cellulose ether or cellulose ether, a centrifugal pump making use of the rotation of an impeller for increasing a lifting height is used for transfer of a slurry of crude water-soluble cellulose ether or crude cellulose ether, which has been dispersed in water for slurrying, after an etherification reaction.
As another means, a volume type rotary pump is used which never gives an excessive shear force to solid particles in the slurry during transfer of the slurry (Japanese Patent Application Unexamined Publication No. 61-34001). In order to efficiently mix the reaction liquid of produced hydroxypropylmethyl cellulose phthalate or hydroxypropylmethyl cellulose acetate succinate with water for industrially producing cellulose derivate particles, a cutter pump is employed (Japanese Patent Application Unexamined Publication No. 64-70502).